Within the field of audio coding it is generally desired to encode an audio signal, e.g. in order to reduce the bit rate for communicating the signal or the storage requirement for storing the signal, without unduly compromising the perceptual quality of the audio signal. This is an important issue when audio signals are to be transmitted via communications channels of limited capacity or when they are to be stored on a storage medium having a limited capacity.
Prior solutions in audio coders that have been suggested to reduce the bitrate of stereo program material include:
‘Intensity stereo’. In this algorithm, high frequencies (typically above 5 kHz) are represented by a single audio signal (i.e., mono), combined with time-varying and frequency-dependent scalefactors.
‘M/S stereo’. In this algorithm, the signal is decomposed into a sum (or mid, or common) and a difference (or side, or uncommon) signal. This decomposition is sometimes combined with principle component analysis or time-varying scalefactors. These signals are then coded independently, either by a transform coder or waveform coder. The amount of information reduction achieved by this algorithm strongly depends on the spatial properties of the source signal. For example, if the source signal is monaural, the difference signal is zero and can be discarded. However, if the correlation of the left and right audio signals is low (which is often the case), this scheme offers only little advantage.
Parametric descriptions of audio signals have gained interest during the last years, especially in the field of audio coding. It has been shown that transmitting (quantized) parameters that describe audio signals requires only little transmission capacity to resynthesize a perceptually equal signal at the receiving end. However, current parametric audio coders focus on coding monaural signals, and stereo signals are often processed as dual mono.
European patent application EP 1 107 232 discloses a method of encoding a stereo signal having an L and an R component, where the stereo signal is represented by one of the stereo components and parametric information capturing phase and level differences of the audio signal. At the decoder, the other stereo component is recovered based on the encoded stereo component and the parametric information.